An aircraft includes an airframe having a powerplant which provides thrust for the aircraft. For large aircraft a common type of powerplant is a gas turbofan engine. The gas turbofan engine includes an engine core and a fan section. The engine core is disposed on the airframe and has a primary flowpath. The fan section is disposed coaxial with and radially outward of the engine core and has a secondary flowpath. A fan cowling envelops the fan section and outwardly bounds the secondary flowpath.
Modern aircraft, and the gas turbofan engines used to power them, are being required to produce increasing amounts of thrust. A consequence of this increased thrust production is an increase in the radial and axial dimensions of the turbofan engine. The radial dimension is generally determined by the fan section and the axial dimension is generally determined by the engine core. The size of the modern turbofan engines has made it more difficult, if not impossible, to ship the engine as a single unit. In addition, the size of the engine makes maintenance and repair work more complex and difficult.
An area of concern with the large gas turbofan engines is the structural mounting of the fan cowling to the engine core. The mounting structure is required to transfer all the operational loads on the fan cowling, such as structural, thrust and aerodynamic loads, to the engine core and thence to the airframe. At the same time, the mounting structure is required to maintain the concentric alignment of the fan cowling and engine core.
One such mounting structure is disclosed in U.S. Pat. No. 3,398,535, entitled "Engine Supporting Structure", issued to Campbell. Campbell discloses mounting structure having a plurality of struts interconnecting an inner ring mounted on the engine core, a mid-span ring, and an outer ring mounted on the fan cowling. The mid-span ring provides increased structural rigidity to the mounting structure. In addition, a plurality of V-struts connect the mid-span ring with points axially downstream on the engine wall in order to transmit axial loads to the engine wall. One drawback to this type of mounting structure is the low efficiency of the load path provided by the multiple rings and V-struts. Another drawback is that the outer casing of the engine core must be stiffened in order to transfer the operational loads to the engine core.
A simple and low cost alternative to the Campbell type mounting structure is a mounting structure having an intermediate case with a plurality of through struts. The through struts are the predominant means to transfer operational loads from the fan cowling to the engine core. Each through strut extends radially from an inner flow surface of the primary flowpath to an outer ring attached to the fan cowling. The through struts provide a simple and efficient load path for the transfer of the operational loads from the fan cowling to the engine core.
A drawback to the through strut concept is the difficulty of separating a powerplant of this type into separate modules for shipping. One solution to this problem was disclosed in U.S. Pat. No. 4,744,214, issued to Monsarrat et al and assigned to Applicants' Assignee, entitled "Engine Modularity". Monsarrat et al discloses structure which permits the turbofan engine to be split into a front portion comprised of the fan cowling, a low pressure compressor, and the intermediate case, and an aft portion comprised of a high pressure compressor and a turbine section. The split occurs by separating a stub shaft from a low pressure rotor shaft, which drives the fan and low pressure compressor, and unbolting the intermediate case from the high pressure compressor.
The above art notwithstanding, scientist and engineers under the direction of Applicants' Assignee are working to develop structure and methods to easily separate gas turbofan type engines into modules for shipping, repair and maintenance.